IC tags utilize radio frequency identification (RFID) technology, and can exchange information with external devices without contacting them. Today's IC tags are increasingly smaller in size and lower in cost, and are being already widely used mainly in the field of physical distribution.
IC tags are beginning to find their applications particularly in the machinery-related field too. For example, the below-identified Patent documents 1 and 2 disclose IC tags attached to component parts of rolling bearings in e.g. wheel bearing assemblies. In each of such IC tags, it is possible to store various kinds of identification information regarding the component part to which the IC tag is attached, such as the type of the component part, the date of manufacture, its production lot, and production history.
By reading the information stored in the IC tag when necessary, such as during storage, during distribution, before use, during use, or after use, of the corresponding component part, anyone can instantly access the identification information regarding this component part. This makes it possible to do away with the old way in which the identification information of a particular component part is searched in a computer or in a book listing machine parts during maintenance or if the component part fails.
But there is one problem in using IC tags in the machinery-related field. That is, many parts used in the machinery-related field are made of metal. If an IC tag is attached to a metal member, its sensitivity tends to significantly decreases due to the influence of the metal member on magnetic fluxes generated by an antenna of the IC tag. If the sensitivity of the IC tag decreases significantly, it becomes difficult for the IC tag to properly communicate with an external reader/writer device, or the communication range significantly decreases.
It is known that if magnetic fluxes generated from the antenna of an IC tag pass through a material having a high conductivity (such as a metal) or a material having a high magnetic permeability (such as iron, nickel, cobalt, their compounds, sendust, carbonyl iron or ferrite (magnetic materials)), sensitivity of the IC tag tends to decrease. Members made of any of these materials are hereinafter referred to as a “metal member”.
Thus, when it is necessary to attach an IC tag to a machine part in the form of a metal member, the IC tag is stuck on the metal member such that the entire antenna protrudes from the surface of the metal member, or the IC tag is embedded in a resin member attached to the metal member, such as a seal member or a sensor case (see e.g. Patent documents 1 and 2).
It is also known, as shown in FIG. 9, to form a countersink 121 in a metal member M so as to embed and fix an IC tag 101 in the metal member M. The diameter of the countersink 121 increases gradually from its bottom 121a toward its opening 121b at the surface of the metal member M.
With this arrangement, as shown by the arrows in FIG. 9, portions of magnetic fluxes generated from the antenna of the IC tag 101 that pass through the metal member M are relative short, so that the magnetic fluxes can readily reach the antenna of an external reader/writer device (not shown) (see e.g. the below-identified Patent document 3).